In the future you should be able to use the --watch flag (to zig build) for this. I believe there is still a component missing that would enable this which is a way for a process spawned by the build runner to hook into the watch system and receive events.
However, that mechanism isn’t really needed for HTTP servers because there’s a simpler way - make that package support serving the files from disk rather than memory (or make an alternative package that does that), in which case --watch already works as-is.
Still, the build system event mechanism is a general-purpose solution to arbitrary build graphs, and so it could be made to work for HTTP servers that serve from in-memory as well, at least as a proof-of-concept. You could imagine a more complicated scenario where it needs to perform some application-specific logic in order to maintain data invariants that go beyond serving files from disk.
Edit: ah looks like I already filed an issue for this: